docs: def RegexField(regex, default=NOTHING, required=True, repr=True, cmp=True,
                 key=None):
      """
      Create new str field on a model.
  
      :param regex: regex validation string (e.g. "[^@]+@[^@]+" for email)
      :param default: any string value
      :param bool required: whether or not the object is invalid if not provided.
      :param bool repr: include this field should appear in object's repr.
      :param bool cmp: include this field in generated comparison.
      :param string key: override name of the value when converted to dict.
      """
      default = _init_fields.init_default(required, default, None)
      validator = _init_fields.init_validator(required, string_types,
                                              validators.regex(regex))
      return attrib(default=default, converter=converters.str_if_not_none,
                    validator=validator, repr=repr, cmp=cmp,
                    metadata=dict(key=key))